Road Test: Mazda 3 2.0 Sport [i-Stop] 5dr

Time for a spot of science. This Mazda3 i-stop is the company’s first crack at stop/start technology. But unlike other similar systems, it's not as simple as you might think. Here's how it works.

When the engine is killed, i-stop knows which piston is in the best position to restart quickest - the one in the ‘combustion stroke' phase, where air and fuel are in the cylinder and ready to be ignited. Sensors pinpoint which cylinder that is, before the mixture is sparked and ignited, forcing the piston down and - along with a small pulse from the starter motor - firing the whole engine into life. It's like placing bicycle pedals just past 12 o'clock, making it easier to push down and move off.

In other words, it starts like any engine, only more precisely and therefore quicker. In fact, when you press the clutch it takes just 0.35 seconds to get going again. By comparison, BMW's stop-starters take 0.7 seconds, as the starter motor has to churn the engine conventionally until it catches.

Phew. That's the theory, but does it actually work? The short answer is yes. It cuts out subtly as you come to a stop and grab neutral, and restarts quickly with less judder than usual.

The problem is keeping it stopped. If you want to run the aircon or wipers or de-misters, it'll only switch off for a few seconds or sometimes not at all. Despite having a second battery to run the starter motor, i-stop is reluctant to do its thing unless you switch everything off. We tested the car in 25-degree heat, selecting a 20-degree cabin temp, and it could only manage about four seconds before calling on the engine again. That's just not good enough in less-than-strenuous conditions - the sort of conditions a BMW system would breeze through.

And then there's the engine itself: a naturally aspirated 2.0-litre with direct-injection. Not the obvious choice for a stop/start system, which would usually be fitted to a diesel or smaller petrol.

Trouble is, i-stop works most efficiently with a direct-injection engine (as less fuel is wasted in the restart process), and Mazda only has two of those on its books - this one, and the 2.3-litre from the CX7. Diesel is incompatible with i-stop in its current state, due to a lack of spark ignition and an extra phase to its combustion cycle. The 2.0-litre manages 41.5mpg and 159g/km, which ain't bad, but doesn't feel substantial enough (consider that a BMW 120i with a conventional stop/start system hits 44.1mpg). Yes, it's an interesting system, and the speed of the restart is impressive. And generally speaking, the Mazda3 is impressive - lovely ride, fun handling and decent interior.

But i-stop just doesn't make sense with this engine. Mazda is working on a solution to that diesel problem and is designing some smaller direct-injection engines too. Only when that happens, and when it works better with the aircon, will i-stop really start to shine.